Steels having zinc additives for improved machinability



Patented June 18, 1963 3,094,440 STEELS HAVING ZINC ADDETWES FOR lMlRGVED MACHINABILITY Elliot S. Nachtman, Oak Park, Ill., assignor to La Salle Steel Company, Hammond, Ind, a corporation of Delaware No Drawing. Filed Aug. 5, 1960, Ser. No. 47,606 7 (llaims. (Cl. 1484) This invention relates to plain carbon and alloy steels and to a method of improving the machinability thereof. It is concerned more particularly with a method of im proving the machinability of plain carbon and alloy steels accompanied with improvements also in physical and mechanical properties of the steel.

It is an object of this invention to produce and to provide a method for producing plain carbon and alloy steels in which the machinability is improved by means easily and simply carried out during normal steel making and it is a related object to improve the machinability of steel with improvement also in other physical and mechanical properties of the steel, such as strength, resistance to wear, resistance to corrosion and toughness.

Numerous attempts have been made in the past to improve the machinability of such plain carbon and alloy steels. To the present, lead, selenium, sulphur and bismuth have been found to provide noticeable improvements in the machinability of steel when added to provide a certain concentration in the steel, but certain undesirable conditions have been associated with their use. For example, improved machinability has been secured by the addition of lead in the manufacture of leaded steels, but since lead is insoluble in the steel, it is difficult to achieve the desirable uniform distribution of the lead in the steel. In addition, lead releases toxic fumes under the processing conditions with the result that extensive precautions must be taken in the use thereof.

Similarly, the distribution of sulphur in the desired concentration in the steel is also difficult to control and unless combined with manganese in substantial amounts, the sulphur will produce an undesirable condition in the steel during rolling, generally referred to as hot shortness. Bismuth and selenium have been used on a minor scale in stainless steel for the purpose of improving machinability. However, the additional cost occasioned by the use of such materials in desirable amounts is a deterrent to their adoption. In addition, these various elements which have been added to steels to improve machinability in many cases cause an undesirable loss in some of the mechanical and physical properties of the steel.

It has been found in accordance with the invention described and claimed in my copending application Serial No. 652,359, filed April 12, 1957, now US. Patent No. 2,789,069, for Method for Improving the Machinability of Stee which is a continuation of Ser. No. 459,527, filed September 30, 1954, now US. Patent No. 2,789,069, that the machinability of plain carbon and alloy steels can be improved materially by the addition of such metals as copper, cadmium, Zinc, cobalt, mercury and tin. The improvements resulting from these metals in such steels are also shown to be further enhanced by the presence of sulphur, phosphorus and nitrogen.

In accordance with the present invention, it has been found even greater improvements in the machinability of steels may be afforded by incorporating into the steel prior to solidification an additive in the form of certain inorganic compounds of the above metals, particularly their sulfides, sulfates, sulfites, tellurides or selenides. These compounds are further enhanced in their ability to improve the machinability of non-austenitic steels by combining therewith, in the finished steel, sulphur, phosphorus and nitrogen. These compounds are for purposes of simplicity referred to hereinafter as additives.

One of the surprising features of the invention resides in the relatively small quantities of the described inorganic compounds of copper, cadmium, zinc, cobalt, mercury and tin required to improve the machinability of certain types of steel. In the case of zinc sulfide, as little as 0.01 percent (expressed as Zn) gives good results although it is preferred to use from 0.02 percent by weight of this compound as well as the other compounds up to their solubility in the particular steel treated.

Improvements in machinability of steel by the use of the compounds of the invention are noticeable with low carbon steel (less than 0.25 percent carbon) as Well as with medium carbon steels (0.25-0.50 percent carbon) and high carbon steels (above 0.5 percent carbon). A typical treated steel of the invention has the following composition as shown below in Table I. Also shown is the same steel without treatment. The additive used was zinc sulfide.

TABLE I II (Treated), percent I (Untreated), percent Steel Carbon. Mona-moan 1 Not detected.

To demonstrate the improvements in machinability achieved by utilizing the compounds of the invention, the following is presented by way of example.

Example Samples of the steels shown in Table I were subjected to tool cutting tests. A measure of the relative machinability was obtained by measuring the wear on the tools after of the parts being machined had been cut under substantially identical conditions.

The force values were for a 0.050 inch depth of cut, a feed of .00028 i.p.r. (inches per revolution), a cutting speed of 200 f.p.m. and no cut-ting fluid was used. The surface finish results were obtained from bars 5 inches long and tapered .005 inch on the radius along the 5 inch length. The width of cut was 0.01 inch in the feed direction while the undeformed chip thickness was. variable from O to 0.01 inch. This procedure makes it possible to measure the finish without the inclusion of feed marks. The cutting speed was 200 f.p.m. for the finish tests and a dry tool was used. The eifective rake angle was 10 in both the force and finish tests.

The finish for steel H is far better than that for steel I. The cutting forces for steel II are unusually low whereas the cutting force values for steel I are relatively high. The tool life results are found to correlate perfectly with cutting force values in this case.

While the above tests have shown the superiority of the zinc sulfide (analyzed in the treated steel as zinc and sulphur) as an additive for carbon steels to improve their machinability, various other compounds of the invention may also be employed to good advantage. Illustrative compounds of the type contemplated for use in improvmg the machinability of carbon steels are the following:

Mercurous sulfide Mercuric selenide Stannic sulfate Stannous sulfate Stannic sulfide Stannous sulfide Stannic selenide Stannous telluride Cadmium selenide Cadmium sulfide Cadmium sulfate Cadmium sulfite Cadmium telluride In the past it has been recognized that residual metals, such as nickel and chromium, have caused marked depreciation in the machinability of steels even when present in small amounts. Contrary to the accepted limitation, it has been found that the deleterious effects of the residual metal on the machinability of steels is reduced by the use of the compounds of this invention. While it is preferred to make use of a steel in which such residual metals are not present, since the metals of the invention are capable then of maximum use, it has been found that the presence of residual metals does not destroy the machinability characteristics of the steel in the presence of the additives of this invention even though less improvement is secured by the use of such additives, nevertheless, in a steel containing the additives in combination with the residual metals provides for better machinability than the same steel without the residual metals and the additives.

It has been found that the improvement in machinability available from the use of the additives of the invention in plain carbon steels and alloy steels can be extended by the addition of other elements with the additives, such for example as phosphorus, sulphur and nitrogen. To some extent, these other elements have been used before because of their beneficial effects on machinability but the improvement which is secured in a system which makes use both of the additive and sulphur together in plain carbon and alloy steels exceeds that which might be expected by way of aggregation to the extent that improvement in machinability is indicative more of a synergistic efiect between these materials in steel. These unexpected results are secured when sulphur is present in amounts ranging from 0.01 to 1.0 percent by weight in the steel and when the additives are present in an amount within the range of 0.06 up to their solubility in steel when in a solid state but in an amount not to exceed 0.6 percent by weight.

When phosphorus is present, the best results are secured when the concentration of phosphorus is within the range 0.01 to 0.2 percent by Weight, and the additives are present within an amount ranging from 0.06 to 0.6 percent by weight or up to their saturation solubility when it is less than .6% by weight, and sulphur, when present, is present in an amount less than 1.0 percent by Weight. Nitrogen in amounts greater than 0.001 percent by weight also improves the machinability of steel when present with the additives alone or in a system which includes the additives and one or more other elements, such as sulphur, phosphorus and nitrogen.

Since many of the additives used in the practice of this invention contain quantities of sulphur, it will be understood that the amount of sulphur expressed above is intended to include the amount of sulphur added to the steel by way of the additive of the invention; that is to say, the sulphur content of the additives is included in the total amount of sulphur additives when such are used in combination with the additives in the practices of the invention.

In the manufacture of steels and other iron base alloys, the additives may be introduced into the metal when in the furnace, ladle or mold. It is an important concept of this invention that when the additives are used to treat steel, normal operating procedures may be employed.

Another important concept of this invention resides in the further discovery that the machinability of steel, for example as measured by the cutting tests previously described, especially steels having the additives in the amounts described, show unexpectedly large improvements in machinability when cold worked. As used herein, the term cold working is meant to include deformation of the steel, as by cold drawing or rolling at room temperature or at elevated temperatures, such as described in U.S. Patent No. 2,767,835; U.S. Patent No. 2,767,836; U.S. Patent No. 2,767,837; and U.S. Patent No. 2,767,838. The most striking elfects by 'way of improved machinability by cold working become apparent with steels containing 0.60 percent carbon or less. Particularly outstanding results are secured with steels having 0.10 to 0.25 percent carbon. To the present, such improvements in machinability are maximized by cold working the steel by reduction in excess of 10 percent in cross-sectional area and generally with reductions of between 20-50 percent depending upon the chemistry of the metal, the size of the raw material being cold worked, and in the case of drawing at elevated temperatures, the temperature of drawing.

When steel is drawn at about room temperature to achieve the desired reduction, strain relieving by heat treatment at a temperature above 550 F. but below the lower critical temperature and preferably within the range of 550950 F. may be desirable. With steels having from 0.2. to 0.4 percent carbon, maximum improvement in machinability is secured by taking heavy drafts which may be followed by strain relieving, especially when copper is present in such steels alone or in a system with sulphur, and with less than 0.15 residual metals composed of nickel, chromium and molybdenum. When working is achieved by drawing at elevated temperatures as described in the aforementioned copcnding applications and issued patents, subsequent strain relieving steps by heat treatment become unnecessary.

While cold working has been found to be desirable to improve the machining characteristics of steel, the combination of cold working steels of the type heretofore described containing the additive alone or containing the additives in a system with sulphur, and/ or nitrogen, offers still further possibilities for providing high levels of machinability together with improved strength, resistance to corrosion, toughness, wear and other mechanical and physical properties without introducing limitations in the processing characteristics of the steel.

The unexpected improvements in machinability secured by metal working is not fully understood. It has been found that steels, particularly those containing carbon in the range of from 0.10 to 0.25 percent, can appreciably be benefited by heavy cold working. These beneficial effects with respect to machinability are enhanced when the steel contains the additives alone or in the presence of phosphorus, sulphur and/ or nitrogen. Whatever the reason, the machinability of steels and iron base alloys, as determined by the energy required for metal separation and by reduction in wear of the turning and forming tools has been found greatly to be improved by modification in the chemistry of the steel to include the additives as an essential component thereof, and further by working of the steel as by cold drawing to secure heavier than normal drafts. Such improvements in machinability have been instrumental in accelerating output of steel products by enabling processing with heavy feeds and higher speeds to increase the production rate while at the same time reducing the time required for replacement and repair to tools and parts. In addition to increased output at reduced costs, the presence of the additives to improve machinability has also provided improvements in other physical and mechanical properties as previously pointed out.

It will be understood that changes in details of formulation, methods of incorporation and processing of the various steels prepared in a manner to provide the characteristics of this invention may be made without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a free machining steel to which a compound of zinc has been incorporated as an additive in an amount up to its saturation solubility in the steel when in the solid state but not in excess of 0.6 percent by weight of the steel, and in which the compound of zinc is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of zinc.

2. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through :a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a steel of the non-austenitic type to which a compound of zinc has been incorporated as an additive in an amount Within the range of 0.06 to 0.6 percent by weight, and in which the compound of zinc is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of zinc, and in which the steel contains less than 0.25 percent by weight residual metals selected from the group consisting of nickel, chromium, vanadium and molybdenum, and subsequently machining the steel to produce parts.

3. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a steel of the non-austenitic type to which a compound of zinc has been incorporated as an additive in an amount of 0.06 to 0.6 percent by Weight, and in which the Zinc is a compound selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of zinc, and which contains 0.01 to 1.0 percent by weight of sulphur and less than 0.25 percent by weight of residual metals selected from the group consisting of nickel, chromium, vanadium and molybdenum, and subsequently machining the steel to produce parts.

4. The metallurgical process for improving the machinability of steel comprising the steps of working the steel to effect reduction in cross-sectional area wherein the steel is a steel of the non-austenitic type to which a compound of zinc has been incorporated as an additive in the amount of 0.06 to 0.6 percent by weight and in which the zinc compound is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of zinc, and which includes 0.01 to 1.0 percent by weight of sulphur, 0.01 to 0.2 percent by weight of phosphorus and less than 0.25 percent by weight of residual metals selected from the group consisting of nickel, chromium, vanadium and molybdenum, and subsequently machining the steel to produce parts.

5. The metallurgical process for improving the machinability of steel comprising the steps of working the steel to effect reduction in cross-sectional area wherein the steel is a free machining steel to which a compound of zinc has been incorporated as an additive in an amount up to its saturation solubility in the steel when in the solid state but not in excess of 0.6 percent by weight of the steel, and in which the compound of zinc is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of zinc, and which includes 0.01 to 1.0 percent by weight of sulphur, 0.01 to 0.2 percent by Weight of phosphorus, and more than 0.001 percent by weight of nitrogen.

6. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel is a steel of the non-austenitic type to which a compound of zinc has been incorporated as an additive in an amount Within the range of 0.06 to 0.6 percent by weight, and in which the compound of zinc is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of zinc, and which includes 0.01 to 1.0 percent by weight of sulphur, 0.01 to 0.2 percent by Weight of phosphorus, more than 0.001 percent by weight of nitrogen, and less than 0.25 percent by weight residual metals selected from the group consisting of nickel, chromium, vanadium and molybdenum, and subsequently machining the steel to produce parts.

7. The metallurgical process for improving the machinability of steel comprising heating the steel to a temperature within the range of 450 F. to the lower critical temperature for the steel composition, advancing the steel through a die to effect reduction in cross-sectiona1 area while the steel is at a temperature Within the range of 450 F. to the lower critical temperature for the steel composition, and wherein the steel is a free machining steel to which a compound of zinc has been incorporated as an additive in an amount within the range of 0.06 to 0.6 percent by weight and in which the compound of zinc is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of zinc.

Metals Handbook, 8th ed. vol. 1, pages 302-307, 1961.

A.S.T.M. Standards, 1949, Part 1, Ferrous Metals, pages 495-498. Published by American Society for Testing Meterials, Philadelphia, Pa. 

1. THE METALLURGICAL PROCESS FOR IMPROVING THE MACHINABILITY OF STEEL COMPRISING THE STEPS OF ADVANCING THE STEEL THROUGH A DIE TO EFFECT REDUCTON IN CROSS-SECTIONAL AREA WHEREIN THE STEEL ADVANCED THROUGH THE DIE IS A FREE MACHINING STEEL TO WHICH A COMPOUND OF ZINC HAS BEEN INCORPORATED AS AN ADDITIVE IN AN AMOUNT UP TO ITS SATURATION SOLUBILITY IN THE STEEL WHEN IN THE SOLID STATE BUT NOT IN EXCESS OF 0.6 PERCENT BY WEIGHT OF THE STEEL, AND IN WHICH THE COMPOUND OF ZINC IS SELECTED FROM THE GROUP CONSISTING OF THE SULFIDES, SULFATES, SULFITES, TELLURIDES AND SELENIDES OF ZINC. 